1. Field of the Invention
The present invention relates to an ultrasonic receiving apparatus and an ultrasonic receiving method for obtaining ultrasonic images by receiving ultrasonic waves.
2. Description of a Related Art
Conventionally, in an ultrasonic imaging apparatus, a one-dimensional sensor array using a piezoelectric element that includes a piezoelectric ceramic represented by PZT (Pb(lead) zirconate titanate) or a macromolecule piezoelectric element such as PVDF (polyvinylidene difluoride) has been generally used as an element (oscillator) for sending and receiving ultrasonic waves. Two-dimensional images in plural cross sections of an object to be inspected are obtained while mechanically shifting the one-dimensional sensor array above, and further, a three-dimensional-image is obtained by synthesizing these two-dimensional images.
However, according to this technique, since a time lag is generated in the shifting direction of the one-dimensional sensor array, cross-sectional images taken at different times are synthesized resulting in a blurred synthesized image. Therefore, the technique is not suitable for taking images of a living organism as an object in ultrasonic echo observation etc. using the ultrasonic imaging apparatus.
In order to obtain high quality three-dimensional images using ultrasonic waves, a two-dimensional sensor capable of obtaining two-dimensional images without shifting the sensor array is required.
If the two-dimensional sensor array is manufactured using PZT or PVDF as above, micro-processing on elements and wiring to a large number of micro-elements are required. However, it is difficult to achieve further miniaturization and integration of elements with the current technology. Even though the difficulties could be overcome, such problems still remain that crosstalk between elements increases, electrodes of microelements become easily broken, and SN-ratio becomes lower due to increase of electric impedance caused by micro-wirings. Therefore, it is difficult to apply the two-dimensional sensor array using PZT or PVDF in practice.
On the other hand, another type of sensor is also known. In a photo-detection type ultrasonic sensor, a received ultrasonic wave signal is converted into an optical signal and detected. As the photo-detection type ultrasonic sensor, a sensor in which a fiber Bragg grating (abbreviated as FBG) is used (see TAKAHASHI et al., National Defense Academy “Underwater Acoustic Sensor with Fiber Bragg Grating”, OPTICAL REVIEW Vol.4, No.6 (1997), pp. 691-694), and a sensor in which a Fabry-Perot resonator (abbreviated as FPR) structure is used (see UNO et al., Tokyo Institute of Technology “Fabrication and Performance of a Fiber Optic Micro-Probe for Megahertz Ultrasonic Field Measurement”, T.IEE Japan, Vol. 118-E, No.11, 1998, pp. 487-492) are reported. Manufacturing a two-dimensional sensor array by using those ultrasonic sensors provides the advantages that electrical wiring to a large number of microelements is not required and satisfactory sensitivity can be obtained.
Further, a photo-detection system ultrasonic sensor having a two-dimensional detection surface is also known. For example, Beard et al., University College London “Transduction Mechanisms of the Fabry-Perot Polymer Film Sensing Concept for Wideband Ultrasound Detection” (IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 46, NO. 6, NOVEMBER 1999, pp. 1575-1582) discloses that a polymer film having a Fabry-Perot structure is used for detecting ultrasonic waves. In the film-like ultrasonic sensor, the cost can be reduced since processing on a large number of micro-elements is not required.
In either case, the photo-detection type ultrasonic sensor utilizes an ultrasonic detecting element having optical reflection characteristics that change by receiving ultrasonic waves. However, in the ultrasonic detecting element, the detection sensitivity widely varies since the optical reflection characteristics also change due to changes in temperature and humidity. Further, in the ultrasonic detecting element having a two-dimensional detecting surface, the detection sensitivity varies since the optical reflection characteristics differ in respective positions on the detecting surface. As described above, the problem in practical use of the photo-detection type ultrasonic sensor is how to control changes in detection sensitivity caused by environmental factors such as temperature and structural factors. For this purpose, a conceivable solution is, for example, to adjust the wavelength of the light output from the light source to the point where the sensitivity of the ultrasonic detecting element is high, however, it is difficult to tune the wavelength of the light of the light source with respect to the reflection characteristics that change very steeply. Another conceivable solution is to allow broadband light to enter the ultrasonic detecting element having different reflection characteristics in its respective positions and filter the reflected light, however, in this case, the constitution of the ultrasonic detecting element becomes complicated and the cost rises. Yet another conceivable solution is to vary the reflection characteristics for respective detection areas of the ultrasonic detecting element, however, also in this case, the constitution of the ultrasonic detecting element becomes complicated and the cost rises.